Air diffusion in rotary hearth calciner

ABSTRACT

Improved operation of a rotary hearth calciner is obtained by introducing the air into the furnace through diffusers or registers spaced above the bed at the circumference of the hearth - the induced air does not impinge directly upon the coke bed but moves as a gentle air front across the hearth, insuring the volatiles being burned near the roof rather than near the coke itself.

i United States Patent [191 Oleszeko et a1,

[ Nov. 12, 1974 AIR DIFFUSION IN ROTARY HEARTH CALCINER.

'[76] Inventors: Thaddeus J. Oleszko, Hoeckstr. F;

Walter H. Groschel, Heckenbergstr. 46, both of 8263 Burghausen, Germany [22] Filed: Sept. 27, 1972 [21] Appl. No.:-292,699

[52] US. Cl 201/32, 201/36, 201/27,

2 [51] Int. Cl Cl0b 21/20 [58] Field of Search 202/102, 103, 104, 151,

[56] References Cited UNITED STATES PATENTS 3,594,286 7/1971 Kemmerer 201/33 CERAMIC BLOCKS WITH HOLES AS DIFFUSERS 2,676,006 4/1954 Martin 432/138 3,434,931 3/1969 Mansfield 201/27 1,296,122 3/1919 Roth 202/98 3,227,627 1/1966 Asquini .1 201/103 Primary Examiner-Wilbur L. Bascomb, Jr.

Assistant E.\aniinerDavid Edwards Attorney, Agent, or Firm.loscph C Herring; Richard C. Willson, Jr.; Jack L. Hummel [571 7 ABSTRACT Improved operation of a rotary hearth calciner is ob tained by introducing the air into the furnace through diffusers or registers spaced above the bed at the circumference of the hearth the induced air does not impinge directly upon the coke bed but moves as a gentle air front across the hearth, insuring the volatiles being burned near the roof rather than near the coke itself.

9 Claims, 7 Drawing Figures PATENTEL IIUV I 2 I974 I SHEET 10F 4 AIR CONVENTIONAL DESIGN I; AIR

DIFFUSER Fig. lb

- Pmtmwv w 3.847. 752

' SHEET 20F 4 CER HOL AMIC BLOCKS WITH ES AS DIFFUSERS HEARTH PATENTELHUYIZIW 3341.152

SHEET 30F 4 II A PATENIEL am 1 21914 sum u [if 4 y 1 AIR DIFFUSION IN' ROTARY HEARTH CALCINER CROSS REFERENCES TO RELATED APPLICATIONS No related U.S. patent applications are known to the inventors.

BACKGROUND OF THE INVENTION 1. Field Of the Invention The present invention relates to the field of DISTIL- LATION: APPARATUS, Directly heated, Rotary, generally classified in U.S. Patent Office Class 202, sub- SUMMARY OF THE INVENTION General Statement of the Invention According to the present invention, the air introduced into the furnace is admitted through a set of diffused air ports which reduce the velocity of the air minimizing the turbulence thus caused within the combustion chamber. This avoids the direct impingement of the air on the bed of calcining coke and also avoids the indirect movement of air downward toward the bed under the influence of turbulence which has been present in conventional furnaces.

The present invention is useful for the coking of a wide variety of calcined solid materials disposed as a bed on the hearth of a rotary hearth furnace. These include coke, e.g., delayed petroleum coke, fluid petroleum coke; coal, e.g., bituminous, anthracite, and other materials which emit combustible volatile matter during calcining and which are harmed by contact of the residuum with oxygen. It is a primary advantage of the present invention that it both reduces the particulate matter emitted from the stack of the furnace and also preserves fixed carbon by reducing both elutriation of fine coke particles and by reducing oxidation of coke.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1a is a diagram showing the conventional air inlet design used in commercial rotary hearth furnaces.

FIG. 1b shows the air diffuser inlet of the present invention in cross sectional view.

FIG. 2 is a perspective cross sectional view of diffusers for use with the present invention showing the brickwork of a preferred embodiment as it is actually installed. The cross sectional area of the smaller holes FIG. 4 shows schematically the pattern of impingement of air which has been found by observation to occur in a rotary=hearth plant employing the conventional air inlets. Elimination of this pattern according to the present invention, substantially reduces carbon loss by both elutriation and oxidation.

FIG. 5a shows schematically a preferred embodiment alternate to that of FIG. 2.

FIG. 5b shows elevation views of alternate configurations of this embodiment.

The cross sectional area of the holes as measured at the interior wall of the furnace is preferably at least 2, more preferably at least 4, and most preferably at least 8 times the cross sectional area as measured at the exterior wall of the furnace.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Starting Materials: The starting materials utilized in the invention can be any of the aforementioned materials capable of being calcined in a rotary hearth furnace which are deleteriously affected by contact with oxygen and which yield oxidizable volatile matter during such calcining.

The volatile matter is oxidized with air (which, as

used herein, can include any oxygen-containing gas).

The air will be admitted in amounts equivalent to from about 0.75 to 1.25, more preferably from about 0.8 to about 1.2, and most preferably from about 0.9 to about 1.1 times the stoichiometric amount necessary to oxidize the volatile combustible matter released during calcining. The air induction points will be from about 6 to about 36, more preferably from about 8 to about 30, and most preferably from about 12 to about 24 inches above the top of the bed of material calcining on the hearth. The incoming air will be passed through diffusers to reduce its velocity (as measured within the furnace about 2 feet away from the inlet), to from about 1 to about 30, more preferably from about 1.5 to about 12, and most preferably from about 2 to about 6 feet per second. The air will be directed either substantially horizontal or somewhat upward in order to eliminate impingement upon the bed of material calcining on the hearth. The temperatures, feed rates of raw coke, or other material to be calcined, and other parameters can be those conventionally employed in commercial rotary hearth calcining furnaces.

Actually the air velocity through the holes of the diffuser is not important, as long as the diffuser diffuses and causes the air velocity in front of the diffuser to have a lazy flow of approximately 2-6 feet per second. The velocity through the holes is substantially higher but as the holes or openings are made smaller, the air jets which come out of the blocks die away quicker. In the block type design diffuser of FIG. 2, these jets die away in the first foot of distance from the blocks. With the same velocity (10 feet/second) through a 1 inch hole and a 6 inch hole, the energy is so much greater in the 6 inch stream that it reaches the bed of coke 58 feet from the inlet point, while a 1 inch jet is hardly noticeable at 1 foot and not at all at 2 feet.

A properly developed shape of the air inlet, that is, a slowly expanding smooth wallbell shape inlet (see FIG. 5) or nozzle can accomplish the same quieting effect at a givendistancefromthe mouth of the bellfor the same nozzle velocity as for 1 inch square edge holes. Anequivalent area andflow quantity bell shape effect.

Apparatus: FIG. 1a shows the conventional air inlet design employed on commercial rotary hearth furnaces such as those employed for the calcining of petroleum coke. FIG. 1b shows the air diffuser inlet of the invention which is substituted for the conventional design. The diffuser is integral with the brickwork so that the wall thickness of the furnace need not be increased.

As shown in FIG. 2, the diffuser can readily be installed into the furnace by modifying one or two rings of the curved bricks which form the periphery of the combustion chamber. By use of perforated bricks such as shown in FIG. 2, a nearly continuous diffusion band encircling the combustion chamber can be provided. Of course, several such rings can be employed where rather large quantities of air must be injected into the furnace during normal operation.

FIG. 3 shows the nearly continuous band of diffusion accomplished by the preferred embodiment of the present invention utilizing the ceramic construction shown in FIG. 2.

FIG. 4 shows the pattern which has been observed when air is fed to a conventional rotary hearth furnace having a bed of coke which is, for some reason, held stationary over a period of time, e.g., because of some interruption in normal operation. During such interruption, it has been noticed that areas of oxidation of coke appear on the surface of the beds. When the diffusers of the present invention are substituted for the conventional air inlets, it is found that these patterns no longer appear even when the bed is held stationary at normal calcining temperature for a period of an hour or more.

EXAMPLE When a rotary hearth calciner identical with that described in US. Pat. No. 3,594,286, except that the air inlets have been revised to the modification shown in FIG. 1b and FIG. 2 of the present invention (with 1 inch style or smaller holes and approximately 3 inch diameter air inlets to the diffuser), is operated as described in that patent, it is found that no pattern similar to that shown in FIG. 4 appears even when the air feed is continued at a time when the hearth has stopped rotating. In contrast, the unmodified furnace prepared according to the aforementioned US patent provides a pattern similar to that of FIG. 4 at times when the hearth is held stationary while air feed is continued and while the temperature is held within the calcining range, e.g., about 2450F. The fixed carbon yields through the use of the invention are improved in excess ofl percent ofthe fixed carbon throughput through the furnace. The coke produced according to the invention is in all respects, equal in quality, size, grain density, and other parameters to that produced according to the techniques and apparatus of the unmodified US. Pat. No. 3,594,286.

Modifications of the Invention It should be understood that the invention is capable of a variety of modifications and variations which will be made apparent to those skilled in the art by a reading of the specification which are to be included within the spirit of the claims appended hereto.

For example, the diffusers can be sintered blocks rather than the perforated blocks shown in the drawings.

What is claimed is:

1. In a method for operating a rotary hearth furnace in which combustible material containing volatiles is deposited on a hearth at a deposit point and is moved across said hearth to an exit point and wherein said volatiles are at least partially burned by contacting with air to form a combustion zone within a furnace chamber at a temperature above the ignition point of the said volatiles, whereby heat is radiated from said combustion zone to said bed of material on said hearth, the improvement comprising in combination introducing said air through diffused outlets, in a substantially horizontal plane at an elevation substantially above the height of said hearth by:

a. introducing said air in an amount equivalent to from about 0.75 to about 1.25 times the amount necessary to oxidize said volatiles,

b. diffusing said air to a velocity of from about I to about 30 feet per second as measured within said furnace chamber about 2 feet in front of said outlets, and

c. introducing said air within a substantially horizontal zone located from about 6 to about 36 inches above the top of said coke bed.

2. A method according to claim 1 wherein said air is diffused to a velocity of from about 1.5 to about l2 feet per second as measured within said furnace chamber about 2 feet in front of said outlets.

3. A method according to claim 1 wherein said air is introduced at a velocity of from about 2 to about 6 feet per second as measured within said furnace chamber about 2 feet in front of said outlets.

4. A method according to claim 1 wherein said air is introduced within a horizontal zone located from about 8 to about 30 inches above the top of the bed of material calcining on the hearth.

5. A method according to claim 1 wherein said air is introduced within a horizontal zone located from about 12 to about 24 inches above the top of the bed of material calcining on the hearth. 

0.75 TO ABOUT 1.25 TIMES THE AMOUNT NECESSARY TO OXIDIZE SAID VOLATILES, B. DIFFUSING SAID AIR TO A VELOCITY OF FROM ABOUT 1 TO ABOUT 30 FEET PER SECOND AS MEASURE WITHIN SAID FURNACE CHAMBER ABOUT 2 FEET IN FRONT OF SAID OUTLETS, AND C. INTRODUCING SAID AIR WITHIN A SUBSTANTIALLY HORIZONTAL ZONE LOCATED FROM ABOUT 6 TO ABOUT 36 INCHES ABOVE THE TOP OF SAID COKE BED.;
 1. IN A METHOD FOR OPERATING A ROTARY HEARTH FURNACE IN WHICH COMBUSTIBLE MATERIAL CONTAINING VOLATILES IS DEPOSITED ON A HEARTH AT A DEPOSIT POINT AND IS MOVED ACROSS SAID HEARTH TO AN EXIT POINT AND WHEREIN SAID VOLATILES ARE AT LEAST PARTIALL BURNED BY CONTACTING WITH AIR TO FORM A COMBUSTION ZONE WITHIN A FURNACE CHAMBER AT A TEMPERATURE ABOVE THE IGNITION POINT OF THE SAID VOLATILES, WHEREBY HEAT IS RADIATED FROM SAID COMBUSTION ZONE TO SAID BED OF MATERIAL ON SAID HEARTH, THE IMPROVEMENT COMPRISING IN COMBINATION INTRODUCINTG SAID AIR THROUGH DIFFUSED OUTLETS, IN A SUBSTANTIALLY HORIZONTAL PLANE AT AN ELEVATION SUBSTANTIALLY ABOVE THE HEIGHT OF SAID HEARTH BY: A. INTRODUCING SAID AIR IN AN AMOUNT EQUIVALENT TO FROM ABOUT
 2. A method according to claim 1 wherein said air is diffused to a velocity of from about 1.5 to about 12 feet per second as measured within said furnace chamber about 2 feet in front of said outlets.
 3. A method according to claim 1 wherein said air is introduced at a velocity of from about 2 to about 6 feet per second as measured within said furnace chamber about 2 feet in front of said outlets.
 4. A method according to claim 1 wherein said air is introduced within a horizontal zone located from about 8 to about 30 inches above the top of the bed of material calcining on the hearth.
 5. A method according to claim 1 wherein said air is introduced within a horizontal zone located from about 12 to about 24 inches above the top of the bed of material calcining on the hearth. 